The invention relates to a pressure holding valve, in particular for pressure-fluid-actuated leveling systems in vehicles.
A known pressure holding valve of this type is embodied as a so-called unblockable check valve (Gotz and Lackmann, "Hydraulik in Theorie und Praxis" [Hydraulics in Theory and Practice], Robert Bosch GmbH, 1981, page 157, or European Patent Document 01 76 679 A1 or German Patent Document 37 10 863 A1). Such pressure holding valves are used wherever, in the position of repose, motion in the blocking direction of the work piston of a work cylinder connected to the work connection has to be avoided on the one hand, an example being the lowering of a load because of the occurrence of leaking oil, and on the other when motion in the previously blocked direction must be executed on command. The opening of the seat valve takes place by means of the control piston, which is acted upon with pressure fluid (liquid or compressed air) via the control line. Because of the sometimes quite major pressure differences and high flow velocities that arise at the valve seat upon opening and closing of the seat valve, the valve member and valve seat are exposed to heavy loads. The only material that can therefore be considered for sealing purposes is metal, preferably steel.
If such holding valves are used in hydraulic leveling systems of motor vehicles, it has been found that unavoidable leakage on the order of magnitude of from 0.5 to 1 cm.sup.3 /min, depending on the valve model, can arise at the seat valve. As a result, after being parked for a relatively long time, several hours, or days, the vehicle body drops down to the stop cushions on the frame, or onto the vehicle axles.